The matter is, indeed, somewhat complicated. I studied in great detail the evacuation of the Mittelbau camps in my PhD dissertation (published 1997). I found out that in two cases the SS guards at a certain time declared the transport "finished" and told the prisoners that they were free. The Reichsdeutsch (!) prisoners, however, being afraid of getting lynched (as it did happen e.g. at Bergen-Belsen and Gunskirchen), preferred to stay with their guards and continued the evacuation march.

Similarly, Helen "Tzipi" Tichauer, a Slovak Jewish prisoner who had been employed at the Birkenau administration, told me in 2002 that, on the eve of the evacuation of Birkenau, the camp commandant called the Reichsdeutsch (!) female prisoners and offered them Entlassungsscheine (documents stating that they were released from camp), so they could leave and be free. After a lengthy discussion together with the other women prisoners, the Reichsdeutsch women decided to reject this offer. They were afraid of falling into the hands of the rapidly advancing Red Army (unfortunately, being women, this fear not without reason) and therefore preferred to stay with the SS and go on transport into the Reich.

Of course these were exceptions and concerned only "Reichsdeutsch" prisoners, i.e. "Aryans" with Reich citizenship, who generally enjoyed some privileges. As a rule, all concentration camps that could be evacuated were evacuated and only those prisoners who were unable to walk were left behind. As the prisoners feared (not without reason) that those left behind would be killed, even very sick prisoners volunteered for joining an evacuation transport. "It was always safest to stay with the crowd," a Camp Dora survivor told me in the 1990s.